Comparison of relativistic effective core potential and
all-electron Dirac-Coulomb calculations of mercury transition
energies by the relativistic coupled-cluster method

N S Mosyagin¹, E Eliav², A V Titov¹ and U Kaldor^{2
  1}Petersburg Nuclear Physics Institute, Gatchina, St Petersburg district 188350, Russia
  ²School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel

 Relativistic effective core potential (RECP) and all-electron Dirac-Coulomb calculations of transition energies for low-lying states of the mercury atom and its ions are carried out with equivalent basis sets by the relativistic coupled-cluster method. RECP results are compared with corresponding all-electron values to estimate the accuracy of different RECP versions. Contributions from correlations of different electron shells to the calculated transition energies are studied. Effects of different nuclear models and of basis set truncation at different orbital angular momenta as well as errors of the Gaussian approximation of the generalized RECP components are reported. We show that at least 34 external electrons of the mercury atom should be correlated and the one-electron basis set should contain up to h-type functions in order to attain consistent agreement within 200 cm^-1 with experimental data.